Clean up batch-preserve-term-settings.exp

Clean up batch-preserve-term-settings.exp

See ChangeLog for details.  No functional change intended.

Tested on x86_64 Debian Stretch by verifying that the gdb.log output
remains unchanged for native, gdbserver and extended-gdbserver.

gdb/testsuite/ChangeLog:

	* gdb.base/batch-preserve-term-settings.exp: Remove top-level
	manipulation of saved_gdbflags.
	(test_terminal_settings_preserved): Remove global declaration of
	the unused variable pagination_prompt.  Remove manipulation of
	saved_gdbflags.  Use a local variable EXTRA_GDBFLAGS instead of
	GDBFLAGS.